The invention relates to a method and an arrangement for a compatible recording and/or reproduction device for the reproduction of information from, and/or for recording onto, optical information carriers having differing information storage densities. The compatible recording and/or reproduction device permits the reproduction of information from, and/or recording on, the hitherto conventional optical information carriers in the form of the standardised CD, mini-disc or MOD as well as from, and/or onto, optical information carriers which exhibit a comparatively higher information storage density i.e. lower track separations and smaller elongations of the pits or storage structures.
Items of information are recorded on the CD or compact disc in digital form as depressions, the so-called pits, that follow a spiral track. The track separation of optical information carriers using the present day norm, amounts to 1.6 .mu.m and the pits have a width of 0.6 .mu.m and a depth of 0.12 .mu.m and the length of a pit or the distance between two pits varies in the range between 0.9 and 3.3 .mu.m, c.f. Philips Tech. Rev, 40, 1982, No. 6, page 156. These standard values are also applicable to the mini-disc. The higher storage capacity of the mini-disc is not attained by a higher storage density of the information carrier but rather, by the compression of the items of information before the recording. The mini-disc corresponds, as regards the construction of the information carrier, to the CD standard values so that the corresponding players do not differ as regards the laser which is used and the scanning device. However, there are, naturally, limits to the amount of data compression that can be effected without loss of information so that one endeavours to increase the storage capacity of the information carrier. This however, necessitates a lower track separation and/or pits having smaller dimensions. As a result, replaying devices of a new type are required which make the optical scanning and the reproduction of information from information carriers of greater storage density possible. For the optical scanning and/or recording of pits of smaller dimensions and having lower track separation, it is necessary to focus the laser beam onto the CD with light spots of smaller dimensions. The dimension of the light spot and also of the numerical aperture of the objective lens as well as of the wavelength of the laser light is thereby determined and thus a smaller light spot diameter or a light spot of smaller dimensions can be achieved by the use of a light source of shorter wavelength. Presently used lasers have a wavelength of 780 nm. However, lasers are already known which produce light having a wavelength of only 680 nm. A quantitative leap is expected from optical frequency doublers, so-called second harmonic generator crystals. They make a halving of the wavelength possible. The power loss thereby arising can be compensated by a higher laser power.
However, the use of a smaller light spot or of a light source of shorter wavelength for the scanning of optical information carriers would lead to information carriers adhering to the present day CD norm not being playable on device of this type. The reason for this, is the principle of destructive interference which is used for the scanning. The principle of destructive interference consists in that an optical scanning of the pits, which are merely formed by depressions in a uniformly reflecting layer, is achieved by focusing the scanning light spot in such a way that the light component reflected from outside a pit is virtually equal to the light component reflected by the depression and thus the light components cancel each other out by destructive interference so that a weakening of the intensity can be evaluated as a scanning signal. There is an interdependence, which is to be maintained, between the dimensions of the pits or of the storage structure and the dimensions of the light spot.
This is also partly applicable to a magneto-optic information carrier, the so-called MOD, in which the storage or reproduction of information is based, in essence, on the rotation of the direction of polarisation. Although the relationship between the dimensions of the magnetic domains and the dimensions of the light spot does not have to be maintained with the degree of accuracy applicable to the pits, a certain interdependence has to be taken into account, even here, for information carriers of differing storage density, especially as regards information carriers having reduced track separation.
Thus, the object of the invention is to develop a recording and/or reproduction device for optical information carriers which is compatible as regards information carriers having differing storage densities despite an interdependence that has to be maintained between the dimensions of the pits or domains and the dimensions of the light spots and which includes a laser emitting light of one wavelength and an objective lens of predetermined focal length.
Although a defocusing of the light beam scanning an optical information carrier, or of writing it in the sense of a recording of information, leads, in known recording and reproduction devices, to a deformation of the light spot reflected onto the photo-detector from the information carrier and this is promptly corrected by means of the focusing control loop, the invention is based, in an advantageous manner, on using a defocusing of the scanning and/or recording beam for a compatible recording and/or reproduction device in order to be able to use optical information carriers of differing generations or of differing storage density in one device. In particular, the endeavour, to increase the quantity of information that is storable on an information carrier, leads to information carriers of greater storage density or to storage structures of lower dimensions. As was already explained, means are known which make it possible to realise CD- and MOD-like information carriers of greater storage density as well as corresponding recording and reproduction device. In this connection, the invention serves, in particular, to overcome the degree of inhibition to the introduction of a generation of devices using information carriers of greater storage density which arises from an assumed incompatibility with the previously used information carriers. A method and an arrangement are specified which permit, in just a single device, the use of information carriers having a storage density corresponding to the present day norm, as well as, of information carriers of greater storage density. This is achieved with a compatible recording and/or reproduction device which is manufactured in such a way that a light source is used, which is adapted to the highest storage density of the information carriers that are to be used as regards wavelength and light spot dimensions, and, the light directed onto the information carrier is defocused by a defocusing means for the enlargement of the light spot when using information carriers of lower storage density. The interdependence which exists between the dimensions of the storage element and the dimensions of the light spot can thereby be maintained even with differently dimensioned storage elements.
In order to have an effect on the beam path in the recording and/or reproduction device in the manner intended, neither a laser producing light of another wavelength nor an objective lens having an altered numerical aperture are used. In accordance with a first embodiment, an effect on the beam path in the recording and/or reproduction device for the defocusing of the light beam directed onto the information carrier is had in that an off-set voltage is used in the focusing control loop as the defocusing means for matching to the storage density of the information carrier. For carrying out the method, an off-set voltage source is provided as the defocusing means in the focusing control loop connected to the photo-detector. Due to the off-set added to the focusing error signal, the objective lens focusing the light beam onto the information carrier is brought, by means of the focusing control loop, into a position which leads to an enlarged light spot on the information carrier and thus can be used for the scanning and/or recording of items of information with an information carrier having a lower storage density. This defocusing is achieved in that the focusing control loop of a recording and or reproduction device guides the objective lens into a position in which a light spot reflected from the information carrier onto the photo-detector is symmetrical with respect to the photo-diodes or photo-segments forming the photo-detector. Since however, an off-set voltage was added to the focusing error signal, this corresponds to a position which deviates from that of focus. A displacement of the working point arising thereby can be counteracted by appropriate dimensioning of the cylindrical lens likewise used in the beam path in known recording and/or reproduction devices. In correspondence with a second embodiment for the manufacture of a compatible recording and/or reproduction device, a means which lengthens the optical path length in the beam path in front of the photo-detector is used as the defocusing means. This may, for example, be a glass plate or a lens. The insertion of a glass plate into the beam path is advantageous vis a vis an altered cylindrical lens for example, since alterations of the position of the cylindrical lens are associated with a displacement of the working point. By having an effect on the beam path in the sense of a lengthening of the optical path length due to the refractive index of the glass plate in the beam path in front of the photo-detector differing from air, a defocusing of the objective lens or enlargement of the light spot suitable for matching to differing information carriers is likewise achieved as a result of the effect of the focusing control loop.
Thus, compatible recording and/or reproduction devices, which are suitable for operation with information carriers of differing storage density, can be manufactured in an advantageous manner.